The present invention relates generally to implantable medical device (IMD) leads for delivering active electrodes to various places in a human body, such as the heart. In particular, the present invention relates to lead conductors that are compatible with radio frequency (RF) fields generated by magnetic resonance imaging (MRI).
Typical leads for use with an IMD, such as an implantable cardioverter defibrillation (ICD) device, deliver multiple conductors to the heart for performing pacing, cardioverting, defibrillating, sensing and monitoring functions. One or more of these conductors typically comprises a multi-filar cable in which nineteen filars are wrapped around a straight central filar. This type of design yields a cable that has good mechanical properties, including flexibility, weldability and high strength. Strength is particularly important for ensuring adequate electrical and mechanical contact between the conductor and an electrode when an electrode is crimped down on the conductor. For example, a good crimp should produce a 2.5 lbf joint. These multi-filar, cables, however, have very low inductance particularly due to the straight central filar. During magnetic resonance imaging, it is necessary to expose the patient and the IMD to a radio-frequency field, which is used to generate the MRI image. Generally, it is desirable for a lead conductor to have increased inductance in order to minimize excitation effects from RF fields generated during magnetic resonance imaging.